Lifting devices are known, which are typically fabricated by manufacturers of tire changing machines and vehicle wheel balancing machines, with the purpose of assisting tire repairers in displacing vehicle wheels from one machine to the other, and in properly positioning these wheels on the balancing shafts of balancing machines, by lifting them above the ground.
Particularly, a known lifting device generally comprises a horizontal platform which is designed to receive a wheel vertically or substantially vertically loaded thereon, when the platform is placed in contact with the ground.
The platform is supported by a lifting system that is supported by a frame and may have a hydraulic or, more frequently, a pneumatic operation, with the thrust of at least one fluid-dynamic actuator.
The fluid-dynamic actuator is supplied with a pressurized fluid, such as air, which comes from a fluid source in the lifting device itself, or is derived from one of the machines with which the lifting device may be combined during its operation.
Typically, once a tire changer has completed the replacement of a tire on a wheel rim and has inflated it again, he/she removes the wheel from the working table top of the tire changing machine and puts it in a vertical position on the platform of the lifting device that has been previously lowered level with the ground.
Then, he/she actuates the fluid-dynamic actuator, by a special control, and the actuator lifts the platform parallel to itself until the latter reaches an elevation at which the central hole of the wheel rim is substantially aligned with the axis of rotation of the horizontal rotating shaft of the balancing machine.
Then, the tire repairer transversely pushes the wheel on the platform toward the balancing shaft until the latter is received in the central hole of the rim.
While the platform is lifted, the tire repairer fits a conical head behind the wheel on the balancing shaft, which head is designed to fasten the wheel against an abutment flange that is typically provided in the area where the balancing shaft is connected to its machine.
While the platform supports the weight of the wheel, the tire repairer will fasten it by tightening the conical head on the balancing shaft, which has an externally threaded section for this purpose.
The conical profile of this head progressively enters the central hole of the rim and simultaneously centers it relative to the longitudinal axis of rotation of the balancing shaft and progressively fastens it in this position against the abutment flange.
Then, the tire repairer lowers the platform again for dynamic wheel balancing.
Once the balancing process has been completed, the tire repairer lifts the platform again until the latter contacts the wheel and supports its weight.
Then, the tire repairer removes the fastening head and slips the wheel off the balancing shaft onto the platform, which is lowered again to unload the balanced wheel.
All operations on the wheel are performed with the assistance of the lifting device, allowing the tire repairer to avoid the efforts caused by vertical movement of the wheel, especially with large and heavy wheels such as those of an industrial, agricultural or heavy-duty vehicle.
A “lifting device, particularly for lifting wheels of vehicles and the like, for wheel balancing and tire removing machines” is known from EP 2 161 235.
This document discloses a scissor-like lifting device, which lifts vehicle wheels without requiring great efforts by operators, and is actuated by a pneumatic linear actuator which comprises a hermetic body in which a piston slides to divide it into two half-chambers.
The liner of the actuator has one end hinged to the base of the lifting device, with a stem projecting out of the opposite end and connected with the piston that has one free end equipped with a roller.
When the actuator extends, this roller simultaneously engages a cam profile embossed in the base and the profile of the bottom surface of one of the two mutually converging rods hinged in scissor-like fashion.
One of the half-chambers, namely the one designed to be supplied for the lifting operation, is connected to an additional pressure accumulator, and when this half-chamber is supplied with pressurized air, the stem progressively comes out of the body and the roller wedges between the two profiles, thereby lifting and pushing up the rod contacted thereby.
A horizontal platform is fixed above the scissor-lie rods, for receiving the objects (vehicle wheels) to be lifted, such platform moving in the same direction as the rod pushed by the roller, i.e. upwards or downwards.
While the first half-chamber of the actuator is being supplied with air, the other half-chamber is discharged for the piston to slide in the direction of fluid pressure.
As soon as the desired elevation is reached, the supply of pressurized air both to the half-chamber and to the additional accumulator is stopped, and the pneumatic circuit is completely closed.
In this condition, the operator may effortlessly move the platform upwards and downwards, and keep it in the selected position.
WO99/40406 discloses an “auxiliary apparatus for handling car wheels in connection with wheel or tire servicing”.
Also in this case, the lifting apparatus consists of a vertically movable platform, which is actuated by a fluid-dynamic actuator.
More particularly, the platform is supported by a pair of extensible arms, which allow the operator to move it toward or away from a machine that controls it.
Like in the previous document, the fluid-dynamic actuator comprises an outer liner with a piston slidably mounted therein to divide it into two half-chambers and again the half-chamber designed to be supplied for lifting the platform is connected to an additional pressurized air accumulator.
Two valves are connected to the half-chambers of this actuator, and they alternately open or close to cause the platform to move up or down.
As soon as the selected elevation is reached, the operator stops the supply of pressurized air from the source to the accumulator and to the half-chamber connected thereto and also closes both control valves: this will close the pneumatic circuit, like in the previous case, whereupon the operator may impart small vertical movements to the platform for optimized alignment thereof with a reference point or axis.
Prior art document U.S. Pat. No. 3,593,980 discloses a “pneumatic balancer” which, like in the previous cases, is actuated by a fluid-dynamic actuator, the latter being divided by the piston into two half-chambers, one of which, i.e. the one in which the piston is designed to run its forward stroke, is connected to an additional accumulator for pressurized fluid (air).
This document discloses a counterbalancing circuit, which automatically balances the amount of pressurized air in the additional accumulator and in the half-chamber of the actuator connected thereto, by adapting it to the weight of the body to be lifted.
This prior art suffers from certain drawbacks.
A first drawback of the device disclosed in EP 2 161 235 is that the additional accumulator is required to have a volume comparable to the maximum volume allowed for the first chamber of the actuator, for effectively limiting pressure reduction therein, as the piston is displaced and the platform is lifted.
For this reason, the amount of compressed air to be introduced into the first half-chamber of the actuator and the additional accumulator connected thereto for lifting a body of a given weight is about twice the amount of air that should be introduced therein if the actuator were not connected to the accumulator, and pressurized air were directly introduced into the first half-chamber of the actuator, progressively as the platform is raised.
Therefore, the power consumed to generate the compressed air required for the lifting operation is also twice as much.
If a fraction of the lifting stroke of the lifting device is only required, the above mentioned ratio of the amount of air introduced into the first half-chamber of the actuator and the additional accumulator to the small amount of air required if it were directly introduced in the first half-chamber only will increase.
A second drawback of this device is that, when it is used for loading or unloading a body on or from a device of a machine that is designed to support it, in order to move the platform back to its rest position and separate it from the body once the latter has been secured to the support device of the machine, pressurized air must be introduced into the second half-chamber of the actuator, opposite to the first half-chamber with respect to the piston, and at the same time the air in the first half-chamber and the additional accumulator must be compressed, whereas the air in the second half-chamber of the actuator must be discharged when the opposite operation must be carried out, to move the platform back to the position in which it supports the body as the latter is removed from the support device of the machine.
This is required to avoid the need of changing the amount of air in the first half-chamber of the actuator, which need would exist if the platform were moved down by discharging air from the first half-chamber and the additional accumulator, because if the amount of air were not changed, the body could no longer be supported by the platform while it is removed from the support device of the machine.
This will further increase consumption of compressed air, and the power required for generating it.
Furthermore, the operator is required to carry out two different procedures to move the platform down to the rest position, depending on whether he/she wants to separate it from the body secured to the support device of the machine by introducing compressed air into the second half-chamber or to lower the body back level with the ground after removing it from the device to unload it from the platform, by discharging air from the first half-chamber of the actuator and the accumulator, and this is a complex arrangement, which increases the probability of incurring errors during operation of the device.
A third drawback is that, during the above described step of forced compression in the first half-chamber of the actuator, pressure in such first half-chamber of the actuator increases, but must be maintained below the maximum pressure admitted in the second half-chamber of the actuator, which is by itself a substantially low value.
Therefore, in order to prevent pressure in the first half-chamber of the actuator and the accumulator from exceeding this limit even when the lifting device is used to lift bodies whose weight is close to the maximum weight that can be lifted by the device, the volume of the additional accumulator must be increased, to limit the percent change of the overall volume contained in the first half-chamber of the actuator and the additional accumulator.
This produces the effect of further increasing the consumption of compressed air and the power required to fill with compressed air both the additional accumulator and the first half-chamber of the actuator.
A further drawback is that the device as taught in EP 2 161 235 requires constant temperature of air in the first half-chamber of the actuator and the additional accumulator, considering that the document assumes that the pressure in the first half-chamber of the actuator and the additional accumulator and the volume enclosed thereby are inversely proportional.
Such constant temperature requirement is only met when volume changes are substantially small or slow, since the walls of the actuator and the accumulator, which are typically thin and made of metal, are good heat conductors and tend to quickly eliminate any minor temperature change with respect to the surrounding environment.
However, the constant temperature requirement is not met, at least temporarily, when large and quick volume changes occur, such as during the forced compression required to separate the platform from the body that has been secured to the support device of the machine or during free expansion, that occurs when the platform is moved back to the position in which it supports the body during removal thereof from the support device of the machine.
Particularly the temperature of air in the first half-chamber of the actuator and the additional accumulator is temporarily but considerably reduced during this second step, thereby entailing a temperature reduction in both, relative to the temperature of the surrounding environment.
This results in a pressure drop in the first half-chamber of the actuator, to a value below the required value, and this pressure drop causes the lifting device to be temporarily unable to support the weight of the body during removal thereof from the support device of the machine, and adds difficulty to these operations.
A further drawback that particularly concerns the auxiliary apparatus as taught in WO99/40406 is that, if the auxiliary apparatus is used for loading and unloading bodies that must be secured to support devices of machines with adequate precision, e.g. for loading and unloading vehicle wheels on and from balancing shafts of balancing machines, the operator is required to accurately locate the initial balance position of the support platform close to the ideal transfer position of the lifted wheel, i.e. with the central hole of the rim horizontally aligned with the balancing shaft of the machine, to reduce the efforts required for minor adjustments of the vertical position of the support platform and the wheel supported thereby.
This requires the first valve means to cause a slow displacement of the support platform.
This requirement is particularly demanding because, according to the weight of the lifted wheel or the height at which the support has to be moved change, and namely according to the diameter of the wheel to be lifted to the elevation coaxial with the balancing shaft of a balancing machine, the operator is required to adjust the amount of air in the first half-chamber and the additional accumulator, by controlling the first valve means.
A further drawback that affects the pneumatic balancer disclosed in U.S. Pat. No. 3,593,980 is that the automatic balancing operation always restores the initial balance position of the system to the same height, which is unacceptable when the height that must be reached by the support platform is required to significantly change according to the size of the lifted body, as is the case of a lifting device that is used for loading and unloading vehicle wheels on and from balancing machines, i.e. when the height that must be reached by the wheel support changes with the outside diameter of wheels.